1. Field of the Invention
The present invention relates to an optical node device and a system including the device.
2. Description of the Related Art
With a rapid increase in IP (Internet Protocol) traffic in recent years, the requirement for construction of a large-capacity, high-flexibility, and low-cost network has been increased. To meet this requirement, the construction of a WDM (wavelength division multiplexing)-based optical ring network is proceeding. In such a network, one wavelength of light is allocated to one path, and an optical node device for adding/dropping an optical signal (i.e., path) having a desired wavelength at an arbitrary node on the network therefore plays an important role. As the optical node device having such a function, an optical ADM (OADM: Optical Add/Drop Multiplexer) is known. The OADM has an all-optical function of adding/dropping an optical signal having a desired wavelength.
FIG. 1 is a block diagram of an optical node device in the prior art. WDM signal light obtained by wavelength division multiplexing a plurality of optical signals having different wavelengths λ1 to λN is input from an input optical transmission line through an optical amplifier into a tunable wavelength selecting element. Optical signals having arbitrary wavelengths (e.g., λ2 and λ3 as shown in FIG. 1) to be dropped in this node are output from a drop port (“drop” shown in FIG. 1) of the tunable wavelength selecting element, and optical signals having arbitrary wavelengths (e.g., λ4 and λ5 as shown in FIG. 1) to be added in this node are input to an add port (“add” shown in FIG. 1) of the tunable wavelength selecting element.
The optical signals output from the drop port are supplied through an optical amplifier to a 1×N optical coupler (1×N CPL), in which the optical signals are split into N channels. The split N channels are next selectively passed through tunable filters (TF) and next received by optical receivers (Rx).
The optical signals to be added in this node are obtained by modulating CW light (continuous-wave light) having desired wavelengths output from tunable laser diodes (TLD), by means of optical modulators (Mod). The resulting plural optical signals are combined by a 1×N optical coupler (1×N CPL), and the resulting light is next supplied through an optical amplifier to the add port of the tunable wavelength selecting element.
The optical signals passing through the tunable wavelength selecting element are combined with the optical signals supplied to the add port, and all the optical signals thus combined are next output from a through port (“through” shown in FIG. 1) of the tunable wavelength selecting element. The optical signals output from the through port are output through an optical amplifier to an output optical transmission line.
In the case of using the optical node device shown in FIG. 1 as each node in a WDM optical network, it is not necessary to terminate (opto/electrical convert or electro/optical convert) the optical signals at all the nodes, but it is sufficient to terminate the optical signals at only the nodes corresponding to a transmitting end and a receiving end for the optical signals (or paths). Accordingly, in comparison with a network based on a SONET (Synchronous Optical NETwork)/SDH (Synchronous Digital Hierarchy) technique requiring the termination of optical signals at all the nodes, a node cost can be greatly reduced.
However, in the case of applying the optical node device to an optical network (e.g., WDM optical ring network) including a closed loop provided by an optical fiber, there is a problem that the oscillation of optical power occurs when the following three conditions are satisfied.                (1) Optical signals are not terminated at each optical node device.        (2) A loop gain in the closed loop is larger than a loop loss.        (3) There exists an optical signal band not added/dropped in any optical node device (e.g., optical amplifier induced ASE noise present in a band between adjacent wavelength channels).        
Referring to FIG. 2, there is shown a WDM optical ring network in which the oscillation of optical power occurs. Nodes A, B, C, and D are arranged clockwise in this order along a closed loop provided by an optical fiber. In this case, optical signals having four wavelengths λ1 to λ4 are added to the node A, and all of the four wavelength channels λ1 to λ4 are dropped from the node D.
As shown in spectrum-1 appearing immediately after the optical signals are output from the node A, the spectrum is not strongly affected by ASE noise generated from the optical node device or an optical amplifier inserted in the optical transmission line. However, as shown in spectrum-2 and spectrum-3, ASE noise generated from a plurality of optical amplifiers (not shown) arranged along the closed loop is accumulated with the transmission of the optical signals. Spectrum-4 shows the spectrum of the optical signals dropped from the node D, and spectrum-5 shows the spectrum of the light passed through the node D. Since all of the optical signals of the four wavelength channels are dropped from the node D, the spectrum-5 includes only the ASE noise in general. When the above-mentioned three conditions are satisfied in such circumstances, the oscillation of optical power occurs (see spectrum-6).
FIG. 3 shows an example of the transmission characteristics of an AOTF (acousto-optic tunable filter) as the tunable wavelength selecting element. More specifically, there are shown the transmission characteristics at the add port, the drop port, and the through port of the AOTF for the four wavelength channels of wavelengths λ1 to λ4. In the transmission characteristic at the through port, the relation between a wavelength spacing (Δλ) and a rejection band width (Δw) is Δw<Δλ. With this transmission characteristic at the through port, the signal components of the wavelengths to be rejected can be sufficiently removed, and coherent crosstalk can therefore be suppressed to thereby minimize the influence of a certain one of the wavelength channels to the other wavelength channels (the wavelength channels passing through the tunable wavelength selecting element).
On the other hand, ASE noise present in any bands other than the signal bands to be rejected, i.e., in each band between the adjacent wavelength channels, is not suppressed, but passed through the tunable wavelength selecting element. When the ASE noise passed through the tunable wavelength selecting element is circulated within a WDM optical ring network satisfying the condition of (loop loss)<(loop gain), the ASE noise is accumulated with the recirculation in the ring network, and this accumulation of the ASE noise is associated with the amplifying operation of the optical amplifiers to finally cause the oscillation of optical power.